Enduring attachments: General Industry Coverage

Think of the oldest piece of machinery your shop uses regularly. Is it 20 years old? Maybe 30 years? Now, consider why it still sees action—probably not just because it still works, but also because the mechanical design is similar enough to modern equivalents that it can perform jobs in a shop today.

That’s a big part of why drill head and tapping equipment manufacturer Rockford-Ettco Procunier still gets repair tickets for tapping attachments, or heads, that date back to its founders’ first entries into the field a century ago.

“The technology is very old,” explained Larry Bull, president of the company’s Procunier arm in Lakeland, Fla. “We actually have a couple of Procunier tapping heads that date back as far as 1919. We still repair tapping attachments that date back to before World War II, which we know because they are made of cast iron rather than aluminum.”

Which all begs the questions: What exactly is a tapping attachment, and why have they been part of the machining industry for so long?

Tapmatic says its RCT50 self-reversing tapping attachment allows for faster cycle times and less wear on both tap and spindle. Image courtesy of Tapmatic.

Synchronous Fix

First, it’s good to understand the different methods of tapping. “Rigid tapping” involves inserting a tap directly into the toolholder of a lathe or mill. Rigid tapping can be done by a master machinist who knows the proper speeds and feeds and how to reverse out of the hole without breaking the tap or the threads. On a CNC machine, the spindle drives the tap into the hole, stops at the bottom and reverses, extracting the tap from the hole at the same speed.

“There’s a lot to be said for the simplicity of rigid tapping,” said Bill Cox, Procunier product engineer. “We have a VMC that we regularly use for rigid tapping because, especially if you only need to tap a couple of holes, it’s just a simple tool change.”

The downside, he continued, is that the spindle must come to a complete stop and reverse while still in the hole, which puts the tap in suboptimal cutting conditions and increases wear on the spindle.

If rigid tapping is not the right method, there are two other general methods to choose from, according to Bob Warfield, CEO of Aptos, Calif.-based CNC training resource and software builder CNCCookbook Inc. There is “floating tapping,” which involves the use of tension-compression or radial-parallel tapholders, and there are self-reversing tapping attachments.

“It all boils down to accurately synchronizing the motion of the tap to the geometry of the thread,” Warfield said. “If you spin that tap at a certain rpm, it had better advance into the hole at the rate the thread’s pitch requires.”

The Tapmatic ASR and RSR self-reversing tapping attachments’ more
compact design is suited for modern CNC lathes. Image courtesy of Tapmatic.

Machines for rigid tapping are accurate enough that they don’t need a tapping holder or attachment to achieve that synchronicity, he explained, though holders and attachments can help increase tool life and decrease cycle times. In fact, simple tapping heads are effectively ancient technology at this point, having been originally developed for manual machines, such as the drill press and the venerable Bridgeport knee mill, with the resulting process commonly referred to as “power tapping.”

A tension-compression tapholder is spring-loaded, he continued, allowing it to compensate for slight axial movements the tap might experience as it enters or exits the hole. “You can push or pull the tap in and out a little bit as it’s working, and because of the flexion inherent to the design, the holder will account for any ‘slop’ that comes about,” he said. Radial-parallel tapholders, as the name implies, offer radial rather than axial float.

Tapping heads feature an internal gearbox and are self-reversing. They allow a machine spindle to continuously rotate in one direction. The tapping attachment rotates and feeds the tap into the hole. Once the desired thread depth is reached, the attachment’s gearing reverses direction and the tap is retracted from the hole without the machine spindle stopping or reversing direction.